The present invention is directed to a process for grafting maleic anhydride or styrene-maleic anhydride complexes onto polyolefins. In particular, the present invention is directed to a process for grafting maleic anhydride or styrene-maleic anhydride onto propylene polymers. More particular, the present invention is directed to a process for grafting maleic anhydride onto polypropylene by the use of a novel catalyst which includes N,N-dialkylethanolamines or N,N-dialkylaminoethyl acrylates (methacrylates) or mixtures thereof.
It has been widely recognized that the specialty chemical industry is the most rapidly growing segment of the U.S. chemical industry. Unlike commodities, specialty chemicals are purchased for the performance and not on chemical specifications. Typically these specialty chemicals are more expensive because they possess unique properties. In the polymer field, it is highly desirable, therefore, to convert plastic commodities into specialties because this would enable one to increase profits. From this point of view, polypropylene plays an important role because of its intrinsic properties such as high melting temperature, low density, high chemical inertness, and its capability to be produced with different morphological and molecular structures. Moreover, it can be modified with the additions of other polymers or mineral fillers and grafted with functional groups. Finally, its low cost and availability in a multiplicity of grades, place polypropylene in an advantageous position in comparison to most other plastic materials.
In order for polypropylene to expand beyond the existing commodity application into the specialty areas, chemical modification for better compatibility with other types of polymers and for better adhesion with various kinds of substrates is necessary. One of the most commonly used approaches for the chemical modification of polypropylene is to incorporate a polar moiety such as a carbonyl group onto the backbone of the polymer. Incorporation of maleic anhydride or acrylic acid onto polypropylene and other polyolefins through graft polymerization has attracted the most attention as a primary means of introducing reactive functionality and publications in this area can be found dating back to about 1960.
Typically, the conventional procedures for grafting maleic anhydride or styrene-maleic anhydride onto propylene have required high reactant levels and/or severe reaction conditions. Moreover, these conventional methods have produced only mediocre graft levels. In fact, no styrene-maleic anhydride technique for grafting onto polypropylene has been commercially practiced to date because of the rigorous exotherm involved in the reaction.
In U.S. Pat. No. 3,414,551 to Reid et al. dated Dec. 3, 1968, the modification of crystalline polypropylene with maleic anhydride is disclosed. The Reid et al. patent discloses a process for reacting propylene with maleic anhydride which does not require a solvent and can be operated at temperatures below the melting point of the polymer. The Reid et al. procedure was a significant step forward in the art with regard to grafting of maleic anhydride onto crystalline polymers, because it appeared to alleviate some of the severe conditions necessary for the reaction to take place. However, the Reid et al. method does not produce significant grafting levels for maleic anhydride on the polymer unless severe reaction conditions are employed. Accordingly, the Reid et al. process has significant room for improvement. The process of the present invention utilizes a novel catalyst component for the grafting of maleic (maleic anhydride) and styrene-maleic complexes [1:1 molar solution (or complex) of styrene (or styrene homologue) with maleic anhydride] onto polypropylene. This process enables one to obtain higher grafting levels for the maleic anhydride or styrene-maleic anhydride complex on the polypropylene at significantly milder reaction conditions. In addition, the process can provide one with a modified polyolefin which does not degrade during the grafting procedure. This, of course, results in an economical process for producing a polymer having improved versatility and potential application over a wide area including adhesives, coextrusion tie layers for multilayer lamanates, metal coatings, polymeric blends, etc.